Many integrated circuit (IC) devices, such as microprocessors, generate enough heat so that heat dissipation is a concern. In many applications, heat transfer elements of the type called heat slugs and heat spreaders are attached directly to the backside of the integrated circuit device to spread and remove heat away from the device. Conventional heat slugs and heat spreaders are typically made of copper, copper/molybdenum alloys or copper/tungsten alloys.
One of the problems with using copper is its high (17.times.10.sup.-6 cm/cm/.degree. C.) coefficient of thermal expansion (CTE) versus the much lower CTE (5-6.times.10.sup.-6 cm/cm/.degree. C.) for silicon and the IC circuit device. This CTE mismatch requires that elastomeric silicone adhesives and uncured thermal greases be used to attach the heat transfer element to the IC device. Doing so prevents the IC subassembly from being destroyed by thermal stresses when undergoing thermal cycles. However, the use of elastomeric silicone adhesives and uncured thermal greases creates reliability problems due to inherently poor bond strength. In addition, silicone-based products are poor thermal conductors and restrict heat transfer away from the IC device.
To alleviate the problems caused by the CTE mismatch, copper/tungsten and copper/molybdenum alloys can be used. The CTE match for these alloys is much closer to that of silicon and ceramic substrates, 6.7-9.0.times.10.sup.-6 cm/cm/.degree. C. versus 5-6.times.10.sup.-6 cm/cm/.degree. C. The closer CTE match permits a more intimate bond, which allows greater thermal conduction. However, these alloys are quite expensive, thus substantially increasing the cost to the ultimate purchaser.
Another problem with conventional, copper-based, heat transfer elements is that they are heavy. While for standalone units this may not be a problem, it is a substantial concern when dealing with computers and other electronic devices for which weight is an important consideration because, for example, they are carried about by the user.
Some electronic devices are low powered and therefore have very low heat outputs. These devices very often have lids molded in place using a polymeric molding compound, typically an epoxy filled with spherical quartz. Although cost-effective, the low thermal conductivity of these molding compounds means that they do not transfer heat very well and thus are totally unsuited for IC devices which generate substantial amounts of heat, such as microprocessors. Rather, lids for high-powered microprocessors are typically made from aluminum or an aluminum/ceramic composite material. The ceramic materials are added to decrease the CTE of the aluminum to a level closer to that of silicon. The lids are typically bonded in place using a hermetic solder, silicon adhesive or other polymeric adhesive.
One of the problems with the conventional aluminum lids is that the CTE is 24.times.10.sup.-6 cm/cm/.degree. C. requiring elastomeric silicon adhesives and uncured thermal greases to be used between the lid and the IC device. This creates the same type of problems as discussed above with reference to heat slugs and heat spreaders. With aluminum/ceramic composite lids the thermal conductivity is relatively low compared to pure aluminum but the manufacturing processes used to make the hardware are more difficult than with aluminum-based hardware.